What is impressions?
Also called views.
Impressions are the total number of times a post was shown on screen, including repeat views by the same person.
Why it matters
Impressions are always equal to or higher than reach. Many impressions from a small reach means the same people saw the post several times. Some platforms now label this number views.
Example
One follower who scrolls past your post three times adds three impressions but only one to reach.